There is an adaptive filter proposed in the related art that uses both the spatial distance and the pixel value difference as arguments in adaptive Gaussian blur (non-patent reference 1). The filter, which assumes the two arguments, is referred to as a bilateral filter. A similar filter is also disclosed in patent reference 1. Such a bilateral filter in the related art uses the product of a weighting coefficient determined by using the argument representing the pixel value difference alone and a weighting coefficient determined by using the argument representing the distance alone.    Patent reference 1: US Patent Application No. 2004/1643    Nonpatent reference 1: C. Tomasi et al. “Bilateral Filtering for Gray and Color Images,” Proceedings of the 1998 IEEE International Conference of Computer Vision, Bombay, India